This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C xc2xa7119 from an application entitled Optical Fiber Amplifier earlier filed in the Korean Industrial Property Office on Nov. 20, 1997, and there duly assigned Ser. No. 97-61595 by that Office.
1. Field of the Invention
The present invention relates to an optical fiber amplifier, and more particularly, to an optical fiber amplifier which obtains gain-flattened amplification characteristics by cascading optical fibers doped with different materials.
2. Description of the Related Art
Wavelength division multiplexing (WDM) transmission has emerged in the latter half of the 1990s as a means for increasing transmission capacity and efficiency by multiplexing and transmitting optical signals of different wavelengths. Thus, an optical fiber amplifier appropriate for the WDM transmission is required. However, an erbium-doped fiber (EDF) amplifier currently and widely in use shows different amplification degrees and noise figures for different wavelengths.
FIG. 1 is a block diagram of a conventional EDF amplifier, comprising a first isolator 100, a pumping laser source 102, a wavelength selective coupler (WSC) 104, an EDF 106, and a second isolator 108.
Their operations will now be described. First, the pumping laser source 102 generates pumping light of a central wavelength of 980 nm. The WSC 104 couples the pumping light to signal light of a 1500 nm band wavelength entering via its input port. The EDF 106 amplifies the signal light according to the pumping light incident from the WSC 104. That is, the pumping light excites erbium ions of ground-state in the EDF, and the signal light is amplified by stimulated emission of the excited erbium. The amplified optical signal is output via the second isolator 108. The first and second isolators 100 and 106 prevent the signal light from being reflected by elements such as input and output connectors 110 and 112 and reentering the EDF.
However, such a conventional optical fiber amplifier has a problem in that signal light of different wavelengths does not provide flattened gains. That is, when input signal channels 1, 2, 3 and 4 of equal powers are incident, the conventional optical fiber amplifier outputs signal channels having different powers at different wavelengths.
In order to solve the above problem, the prior art uses filters or changes the configuration of EDFs. However, use of the filters increases costs and increases the size of the conventional optical fiber amplifier. The above-described methods result in a reduction in gain since they flatten output powers on the basis of the minimum power among the output powers at different wavelengths. The use of EDFs doped with a material having a different basic composition, such as fluoride, enlarges a gain flattening band, but reduces gains and shows characteristics unstable due to environmental influences.
To solve the above problems, it is an object of the present invention to provide an optical fiber amplifier having a flattened gain by cascading EDFs to which different materials are added.
Accordingly, to achieve the above object, there is provided an optical fiber amplifier comprising: a first optical fiber doped with erbium and phosphorous, for amplifying signal light excited by the erbium; a second optical fiber connected to one end of the first optical fiber, doped with erbium and aluminum, and having a gain spectrum slope opposite to the first optical fiber according to population inversion of the erbium, for amplifying signal light output by the first optical fiber; a pumping laser source connected to the other end of the first optical fiber, for exciting the erbium of the first and second optical fibers; and a light coupler for coupling pumping light from the pumping laser source to the signal light and outputting the resultant light to the first optical fiber.